1. Field of the Invention
This invention relates to the field of shell and tube heat exchangers. More specifically, this invention relates to shell and tube heat exchangers as are used in nuclear powered electricity producing plants.
In a nuclear power plant super-heated power steam is commonly employed to drive a turbine which is connected to an electric generator. After passage through one or several stages of the turbine, water is likely to condense in the stream of power steam. Hence, the power steam is reheated after passing through a turbine stage before the power steam is permitted to return to the next successive turbine stage. The power steam, after performing its work on the first turbine stage, is first passed through a water droplet separator and then reheated for instance in a shell and tube heat exchanger before returning to the next successive stage in the turbine.
2. Description of the Prior Art
Various shell and tube heat exchangers are known in the art, as shown by, e.g., U.S. Pat. Nos. 1,883,605 to Davy, and 3,443,548 to Rich et al.
A shell and tube heat exchanger of the type known to the prior art for reheating steam is depicted in FIGS. 1 and 2 with a cylindrical shell 1 having power steam inlet PSi and outlet PSo and enclosing a heating system formed of a cylindrical arrangement of longitudinally aligned heating tubes 2 which are supplied with heating steam from an inlet HSi through a main inlet manifold 4i, inlet feeders 5i and inlet subheaders 3i. The heating steam, after passing through heating tubes 2, passes through outlet subheaders 3o, outlet feeders 5o and main outlet manifold 4o to heating steam outlet HSo. With such shell and tube heat exchangers, the inclusion of condensed water in the power steam tends to produce heat starved heating tubes in a degradating process that tends to repeat itself in a cyclic manner.
For example, a sudden increase in the load on the turbine reduces the power steam temperature to a level where the condensed water in the stream is not sufficiently removed by the separator and permitted to enter the reheater shell and tube heat exchanger. The water-containing power steam, as it passes through the shell and tube heat exchanger, contacts a heating tube. This water-contacting heating tube then commences to operate as an evaporator rather than as a steam heater. The evaporator function of the heating tube causes the latter to operate at substantially constant temperature and draws an unusually large amount of heat from the heating steam within the heating tube with the result that heating steam condenses in the heating tube. As the condensate in the heating tube continues to build up, it draws less heating steam and effectively becomes heat starved. The build up of condensate in a heating tube is reflected in pressure losses in the feeder which supplies heating steam to the heat starved condensate-containing heating tube. In effect pressure loss in the feeders introduces non-uniform heating of the power steam as it passes through the shell and introduces "cold strands" of power steam. These cold strands are not adequately dispersed throughout the stream of power steam by the time the next successive turbine stage is reached and water droplets are likely to enter the turbine causing damage to turbine blades.
In effect, therefore, the presence of water in the power steam introduces an unbalanced condition on both the shell and tube sides of the heat exchanger.